The present invention relates, in general, to solid state sensors, and more particularly, to solid state sensors having a micromachined capacitor structure.
Many micromachined devices are now well known, including force, acceleration, and pressure sensors. The term "micromachined" is used because the devices employ mechanical structures and spacings which are as small as a few tenths of a micrometer. The small dimensions are achieved by photolithographic and etching techniques similar to that used in integrated circuit manufacturing. Usually, many devices are manufactured on a single substrate. Often, a silicon substrate is used.
Most often, micromachined sensors use piezoresistive properties of silicon to generate a signal. Alternatively, capacitor plates can be formed on the substrate so that at least one capacitor plate can move with respect to another capacitor plate. The relative movement in response to pressure or acceleration changes the capacitance of the structure. This change in capacitance is detected as an output signal.
Unfortunately, prior micromachined capacitor structures suffer from a number of limitations which raise the cost of manufacture, limit accuracy, and preclude their use in many applications. For example, the moving capacitor plate can easily be distorted by residual stress incorporated during manufacture or from wear during extreme shock. Because the spacing between the capacitor plates is in the order of micrometers, these distortions often led to capacitor plates touching during operation, risking permanent damage. Also, capacitive structures have been developed having dynamic closed loop control systems to accurately position the movable or dynamic capacitor plate with respect to fixed capacitor plates. Closed loop systems only work well when movement of the dynamic capacitor plate is constrained within a narrow range.
Prior capacitor structures were usually designed as cantilevers with one end anchored to a substrate and another end free to swing in relation to a fixed capacitor plate. Because the free end of the cantilever structure swings more as the movable capacitor plate becomes larger, designs in which the swing distance was unconstrained had to have small capacitor plates. The smaller capacitor plate in turn lowers sensitivity of the device, precluding use in sensitive circuits.
What is needed is a micromachined capacitor structure and method for making it which provides travel limits on the dynamic capacitor plate to prevent electrical latching or mechanical damage during extreme shock.